This invention relates to a closed loop, convection cooled gas turbine bucket and to a method for cooling the platform and airfoil fillet region of the bucket.
The technology of gas turbine bucket design is continually improving. Current state-of-the-art designs employ advanced closed loop cooling systems, higher firing temperatures and new materials to achieve higher thermal efficiency. Coincident with these advances, there is an ever increasing need to design components to avoid crack initiation and subsequent coolant loss due to low cycle fatigue.
Low cycle fatigue (LCF) is a failure mechanism common to all gas turbine buckets. It is defined as damage incurred by the cyclic reversed plastic flow of metal in a component exposed to fewer than 10,000 load cycles. Low cycle fatigue stress is a function of both the stress within the section as well as the temperature. The stress may come from mechanical loads such as pressure, gas bending, or centrifugal force, or the stress may be thermally induced, created by the difference in metal temperatures between various regions and the geometric constraints between these regions. Minimizing thermal gradients within a structure is key to reducing LCF damage.
In advanced gas turbine cooled bucket designs, particularly those with thermal barrier coatings, the airfoil bulk temperature tends to run cooler than the platform at the base of the airfoil, creating a thermal stress in the platform and airfoil fillet region on the pressure side of the airfoil (where the airfoil portion joins the platform). Adequate cooling of this region is necessary to reduce the stress and to improve the low cycle fatigue life.
During the production of the present bucket casting, the crossover core that generates the hollow cavity through which coolant is delivered to the machined trailing edge holes is locked into the shell system at the root of the bucket. The crossover core is also held by the shell at two mid-span locations (reference crossover core supports denoted in FIG. 1), and again at another location near the top of the crossover core.
It is critical to control the location of the top of the core since it is this location that forms a xe2x80x9ctargetxe2x80x9d for drilling the trailing edge cooling holes in the airfoil portion of the bucket. These machined trailing edge cooling holes must intersect the top of this core in order for coolant to flow through these holes and provide cooling to the airfoil trailing edge. One of the root causes of poor position control is inherent in the design. Specifically, since there is a difference in thermal expansion between the ceramic shell and ceramic core used in the casting process, and due to the relatively long length of the crossover core (approximately 12 inches) the crossover core is xe2x80x9cpulledxe2x80x9d by its root end where it is locked in the shell. Attempts to lock this design at the tip have failed due to the fragility of the core.
This invention seeks to improve the low cycle fatigue capability of turbine buckets through use of an improved cooling system that is also more producible and cost effective. The design and manufacturing improvements are summarized below.
In terms of design, the crossover passage is opened to the cooling passage in the shank portion of the bucket at a location close to the underside of the platform, and then runs along the underside of the platform towards the trailing edge of the airfoil. This arrangement cools both the platform and the airfoil fillet region. For a second stage bucket, the flow direction can run from the aft portion of the bucket toward the leading edge where the flow enters a radially extending cooling passage in the airfoil portion of the bucket.
This design change means that the total height of the core used in the manufacture of the bucket can be shortened to reduce the amount of thermal mismatch. The redesigned crossover core can be locked in the shell at the forward or radially outer core end, thus eliminating the prior core end location problem. Since the crossover core will bump against the main body core, there is also no concern with respect to relative radial movement of the two cores. The crossover core will be allowed to float at the aft or radially inner core location. Since the core will be completely encapsulated by shell, however, and in close proximity to the platform, it is anticipated that relative movement between the core and the platform will be reduced, and thus dimensional control improved. A further benefit of this design will be lighter weight, chiefly due to the reduced size of the central rib in the shank portion of the bucket.
The design concept may also be implemented as a post cast fabrication rather than cast. In any event, the manufacturing process employed to produce the new bucket platform cooling circuit is not regarded as part of the invention per se.
The internal heat transfer coefficients of the new crossover passage design may be optimized either through tuning the cross sectional area or wetted perimeter, thus controlling flow velocity and heat transfer coefficient. Further, the passage may be locally turbulated to increase the local heat transfer coefficients without unnecessarily increasing pressure loss and heat pickup throughout the passage.
Alternative designs within the scope of the invention permit the cooling of virtually any region of the platform by simply re-routing the crossover passage along the underside of the platform. It is also contemplated that cooling steam be metered into the trailing edge holes by the cooling holes themselves. In applications where there are no trailing edge holes to meter the cooling flow, the amount of flow that would bypass the main cooling circuit would be too great given the size limitations that would be placed on the minimum cross sectional area of the crossover passage in order to achieve adequate core producibility. Accordingly, for such applications, a separate means for metering the flow into the trailing edge holes is provided.
In its broader aspects, therefore, the present invention relates to a closed circuit steam cooling arrangement in a gas turbine bucket having an airfoil portion joined to a platform along a fillet region and where a steam cooling supply passage is adapted to supply cooling steam to the airfoil portion of the bucket, and which also includes a crossover passage extending adjacent and substantially parallel to the platform.
In another aspect, the invention relates to a turbine bucket comprising an airfoil portion having leading and trailing edges; at least one radially extending cooling passage within the airfoil portion, the airfoil portion joined to a platform at a radially inner end of the airfoil portion; a dovetail mounting portion enclosing a cooling medium supply passage; and, a crossover passage in fluid communication with the cooling medium supply passage and with at least one radially extending cooling passage, the crossover passage having a portion extending along and substantially parallel to an underside surface of the platform.